Tooth-colored resin composites are being widely used for direct restoration of teeth. (Moszner, N., Hirt, T., “New Polymer-Chemical Developments in Clinical Dental Polymer Materials: Enamel-Dentin Adhesives and Restorative Composites,” Journal of Polymer Science Part A: Polymer Chemistry, 2012, 50(21), 4369-4402; Moszner, N., Salz, U., “Recent Developments of New Components for Dental Adhesives and Composites,” Macromolecular Materials and Engineering, 2007, 292: 245-271.) “The increasing use of dental composites restorative systems and the associated dentin/enamel bonding agents is approaching approximately 65-70% of dental restorations placed in USA,” (2012 NIH/NIDCR Funding Opportunity Announcement: Design and Development of Novel Dental Composite Restorative Systems (U01), RFA-DE-13-001, Sep. 12, 2012; American Dental Association (ADA), “The 1999 Survey of Dental Services Rendered,” Chicago, Ill., ADA Survey Center, 2002.) However, recent reports show that half of current dental restorations fail within 10 years. (Frost, P. M., “An Audit on The Placement and Replacement of Restorations in a General Dental Practice,” Primary Dental Care, 2002, 9: 31-36; National Institute of Dental and Craniofacial Research (NIDCR) Announcement No. 13-DE-102, Dental Resin Composites and Caries, Mar. 5, 2009.) The replacement of the failed restorations accounts for about 50-70% of all restorations work (ca $5 billion/year cost) in the USA. (Jokstad, A., Bayne, S., Blunck, U. Tyas, M., Wilson, N., “Quality of Dental Restorations. FDI Commission Projects 2-95,” International Dental Journal, 2001, 51: 117-158; Deligeorgi, V., Mjor, I. A., Wilson, N. H., “An Overview of Reasons for the Placement and Replacement of Restorations,” Primary Dental Care, 2001, 8: 5-11.)
One critical problem of the current composites is the balk fracture due to their degradation of the polymeric matrix and/or interface in oral environment (Soncini, J. A., Maserejian, N. M., Trachtenberg, G. H., Tavares, M., Hayes, C., “The Longevity of Amalgam Versus Compomer/Composite Restorations in Posterior Primary and Permanent Teeth,” Journal of the American Dental Association, 2007, 138: 763-772; Bernardo, M. Luis, H., Martin, M. D., Leroux, B. G., Rue, T., Leitão, J., DeRouen, T. A., “Survival and Reasons for Failure of Amalgam Versus Composite Posterior Restorations Placed in a Randomized Clinical Trial,” Journal of the American Dental Association, 2007, 138: 775-783), leading to early failure and short lifespan (Ferracane, J. L., Hopkin, J. K., Condon, J. R., “Properties of Heat-Treated Composites After Aging in Water,” Dental Materials, 1995, 11: 354-358; Ferracane, J. L., “Current Trends in Dental Composites,” Critical Reviews in Oral Biology & Medicine, 1995, 6: 302-318), and release of potential toxic compounds from composites. (Gonçalves, T. S., Morganti, M. A., Campos, L. C., Rizzatto, S. M., Menezes, L. M., “Allergy to Auto-Polymerized Acrylic Resin in an Orthodontic Patient.” American Journal of Orthodontics and Dentofacial Orthopedics, 2006, 129: 431-435.) Therefore, it is imperative to develop new composite system, which is hydrolytically stable, thus they can avoid early failure, prolong clinical service lifespan, and save the replacement cost.
Currently used dental composites include an initiator and three major components: polymeric matrix, glass filler, and silane coupling agent. Since introduced by Bowen in the early 1960s (Bowen, R. L., “Dental Filling Material Comprising Vinyl-Silane Treated Fused Silica and a Binder Consisting of the Reaction Product of Bisphenol and Glycidyl Methacrylate,” U.S. Pat. No. 3,066,112, 1962; Bowen, R. L., “Properties of a Silica-Reinforced Polymer for Dental Restoration,” Journal of the American Dental Association, 1963, 66: 57-64), the resin chemistry for current composites on the market has not changed: bisphenol-A-diglycidyl dimethacrylate (BisGMA), urethane dimethacrylate (UDMA) and Methylene glycol dimethacrylate (TEGDMA). (O'Brien, W. J., Dental Materials and Their Selection, Edition 3, Quintessence Publishing Company, Inc., Polymeric Restorative Materials, 2002; 113-131; Gladwin, M., Bagby, M., Clinical Aspects of Dental Materials: Theory, Practice, and Cases, 3rd Edition, Lippincott, Williams & Wilkins, Baltimore, 2009.)
During clinical application, these monomers and coupling agent (e.g., 3-methacryloxypropyltrimethyoxy silane, MPS) on the surface of the filler will form a cross-linked polymeric matrix, in which the ester bonds plays the roles as “bridge” within the polymeric matrix and the interface between matrix and filler (FIG. 1).
However, these “bridge” ester bonds may degrade in the oral cavity, which can be accelerated by acid (such as bacteria acid and soft drinks), or salivary enzymes. (Santerre, J. P., Shajii, L., Leung, B. W., “Relation of Dental Composite Formulations to Their Degradation and the Release of Hydrolyzed Polymeric-Resin-Derived Products, Critical Reviews in Oral Biology & Medicine, 2001, 12: 136-151.) Hydrolysis of these “bridge” esters will breakdown the polymer backbone and the interface, thus reducing the mechanical strength (FIG. 1). (Finer Y., Santerre, J. P., “Salivary Esterase Activity and its Association with the Biodegradation of Dental Composites,” Journal of Dental Research, 2004, 83: 22-26; Yourtee, D. M., Smith, R. E., Russo, K. A., Burmaster, S, Cannon, J. M., Eick, J. D., Kostoryz, E. L., “The Stability of Methacrylate Biomaterials When Enzyme Challenged: Kinetic and Systematic Evaluations,” Journal of Biomedical Materials Research, 2001, 57: 522-531.)